Fluoropolymer resins, and especially perfluoropolymer resins, are known for their low surface energy and non-stick properties as well as thermal and chemical resistance. However, fluoropolymer coatings often show weak abrasion resistance and lower hardness. It has long been desirable to achieve longer wearing non-stick polymer coatings on metal substrates. Of particular concern to achieving coated substrates with longer service life is the coated substrate's ability to withstand abrasion. Abrasion refers to the amount of coating that is worn away as may occur by rubbing or sanding wherein the coating fibrillates and breaks away or shreds from the surface. In damaging a coated substrate, an initial scratch may be followed by abrasion, in that a knife which causes plastic deformation of the coating, may also lead to the formation of fibrils which are subsequently worn away. Such defects additionally compromise corrosion resistance.
A non-stick coating is optimized for release so as to prevent food particles from sticking to it after cooking or to facilitate low friction sliding contact in other applications. However, the attributes that result in desirable non-stick properties also result in difficulties in getting non-stick coatings to adhere well to the substrate. Good adhesion to the substrate is viewed as a pre-requisite for both good abrasion resistance and good corrosion resistance.
Generally in the art, adhesion has been achieved by roughening the metal substrate prior to application of the non-stick coating so that mechanical bonding will assist chemical interaction of binders in a primer layer in promoting adhesion. Typical roughening includes acid-etching, sanding, grit-blasting, brushing and baking a rough layer of glass, ceramic or enamel frit onto the substrate. Such treatments are a partial but insufficient solution to the adhesion problem.
Prior efforts at achieving scratch-resistant and abrasion resistant coatings have included using harder auxiliary heat resistant resins along with perfluorocarbon polymers, or using fillers such as mica and aluminum flake. However, adding fillers (inorganic or organic) into the primer layer may result in weak adhesion to the substrate or to the upper layer or both, or, the non-stick property may be weakened if fillers are added to the top coat. And addition of fluororesin into the primer layer may result in weak adhesion to the substrate, or weaken the intercoat adhesion for the midcoat or topcoat if fluororesins are added to the midcoat or top coat layer.
U.S. Pat. No. 6,761,964 (to Tannenbaum) discloses a coated substrate having a non-stick coating comprising a primer layer adhered to the substrate wherein the primer layer comprises inorganic film hardener including large ceramic particles essentially encapsulated by the primer layer and extending into the midcoat layer.